Is It Real or Is It Memorex?

So, how does it work when a writer wants to use the names of real things/people/books/etc. in her fictional world? Does she need to get permission?

For example, if my heroine is a journalist, can she write for Glamour or Cosmopolitan or Vanity Fair? Can my ex-Secret Service hero have worked on President Clinton's security detail? Can my newly famous starlet heroine buy the Bel Air mansion right next door to Brad Pitt and Angelina Jolie (not that they own a Bel Air mansion, but I'm flying off the cuff here)? Can it be a United flight that gets hijacked, the NBC television studio that gets taken over by a crazed gunman, or the New York Times that spills headlines warning of a deadly plague heading toward the U.S.?

How close to reality can I skim before I hit legal issues? When and where do I have to give credit? When do the real people involved with these real places/people/institutions whatever crawl out of the woodwork to cry foul and nail me for the faker that I am?

And how bad is it to offer up substitutes?

If my chick lit heroine writes for Dazzle or Jet Setter or Blue Blood Monthly magazine, does the reader mentally understand the parallel to the real-world counterpart? Does having worked the Secret Service detail for ex-president Martin Smith carry the same sense of competence one would expect simply in seeing the words Secret Service? And does owning the Bel Air mansion next door to Rake Famous and Fiona Fabulous, the fictional reigning couple of fictional Hollywood, denote the same sense of fairytale and voyeurism I imagine I would feel if I ever entered that world?

I really don't know. But I need to. Quickly.